Mumbai. Shanghai. Tokyo. Beijing. Megacities housed about 12 percent of urban dwellers as of 2014. This is according to the United Nations. Urban areas are where people live close to each other. Megacities are areas with populations of 10 million or more. Here are five things to know about what is likely the wave of the urban future.

There could be 41 megacities by 2030.

In 2014, the United Nations released a report. It was on the world's urbanization prospects. It showed amazing growth in terms of megacities. There were just 10 megacities in 1990. By 2014, that number had spiked to 28. But as urban populations continue to swell, the UN projects that humans will merge into a whopping 41 megacity regions. It should happen by 2030. Look for growing megacities in the developing world, writes the UN. An additional 2.5 billion people are projected to be added to the world's urban population. This is expected by 2050.

Megacities can wreak havoc on the environment...

With all those people living in urban areas, it's perhaps not surprising that megacities have an environmental impact. But the sheer scale of that impact is sobering. In a 2015 report on megacities' environmental impact, researchers from the United Kingdom and Germany rounded up existing research on the footprint of megacities. Their review found that megacities' most intense impact will be on the atmosphere. The world's largest cities emit huge amounts of greenhouse gases. And, they give off large amounts of atmospheric pollutants. The huge cities are expected to emit even more as time goes on. In 2005, megacities emitted 12 percent of all of the carbon dioxide on Earth. That number is expected to grow as the century progresses.

...but they could one day be a force for environmental good.

The environmental outlook for big cities isn't all bad. The German Advisory Council on Global Change just released a report. It suggests that cities facing a huge influx of residents could transform into a source of environmental transformation. The report suggests that huge cities lessen the environmental blow of urbanization. The cities do this by distributing urban resources across surrounding regions. But the report also points to the use of renewable energy and adaptation to climate change. It says these are areas in which megacities could lead environmental healing.

One day, megacities could be more important than countries.

Could the country one day be passÇ? Perhaps, writes Parag Khanna for Quartz. Khanna's new book is titled Connectography. It explores how the planet is connected. He argues that megacities will soon be more important than the arbitrary national borders that once divided countries. As infrastructure continues to integrate urban areas, writes Khanna, megacities will attract even more wealth and population. One day, perhaps, the huge cities may recreate the world map.

They even have their own art exhibition.

Are you convinced yet that megacities are a mega player? You could check out Megacities Asia. It is a new art exhibit. It can be seen at the Museum of Fine Arts Boston. The exhibit features diverse art from 11 renowned artists. They come from different Asian cities, like Seoul and Delhi. You can see a backpack-constructed snake. It pays tribute to over 5,000 schoolchildren who died in a Chinese earthquake. And there is a sculpture made entirely of discarded plastic bags from Mumbai. The entire exhibit is as expansive and lively as megacities themselves. It runs until July 17. That is plenty of time for you to get a glimpse of some of the world's biggest cities without grabbing your passport.